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Vol. 30, Issue 9, 1029-1034, September 2002
Molecular Toxicology Group, School of Biomedical and Life Sciences,
University of Surrey, Guildford, United Kingdom
CYP3A4 is the most abundant cytochrome P450 (P450) in human
liver, comprising approximately 30% of the total liver P450 content. This enzyme has an important role in steroid catabolism and metabolism of foreign compounds, with the majority of pharmaceutical compounds being substrates for CYP3A4. The molecular mechanisms that underlie transcriptional activation of CYP3A4 are complex with many steroid hormone nuclear receptors, including glucocorticoid receptor, pregnane
X receptor (PXR), vitamin D receptor, and constitutive androstane
receptor, playing roles. Nowhere is this more evident than in the
induction of CYP3A4 gene expression by glucocorticoids. CYP3A genes
lack a consensus glucocorticoid receptor response element and yet are
highly induced by classical glucocorticoids such as hydrocortisone and
dexamethasone. Recent evidence has demonstrated that glucocorticoids
are ligands for the orphan nuclear receptor PXR, and induction of CYP3A
genes by glucocorticoids may occur primarily through PXR interactions.
In this paper, we present a mutant that disrupts a
hepatocyte-nuclear-factor-3/CCAAT-enhancer binding protein
binding
site in the CYP3A4 proximal promoter. This mutation disrupts induction
of a reporter gene construct by the glucocorticoids dexamethasone and
hydrocortisone; yet induction by the potent PXR ligand rifampicin is
unaffected. Such data provides strong evidence that glucocorticoids
induce CYP3A4 gene expression both through the established
PXR-dependent pathway but also through a PXR-independent pathway.
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